Interferometry may be used to determine a spatial property of an object. Typical spatial properties include a surface topography or location of the object with respect to some reference. For objects including a thick film overlying an opaque substrate the data may include two interference patterns resulting, respectively, from the substrate-film interface and film-air interface. If the interfaces are widely spaced, the data can sometimes provide independent information about the substrate surface and film-air interface. As the overlying film becomes thinner, the respective interference patterns begin to overlap and distort one another. Such overlapped interference patterns can provide erroneous spatial information regarding the substrate surface and film-air interface.